Reduction-reoxidation type semiconducting ceramic capacitor

ABSTRACT

An improved reduction-reoxidation type semiconducting ceramic capacitor and the method for producing the same are disclosed. The main components of the substrate of the capacitor are CaTiO 3 , SrTiO 3  and Bi 2  O 3 .xTiO 2 . Said substrate also contains at least one member selected from the group consisting of manganese, cobalt, nickel, chromium, vanadium, niobium, tantalum, lanthanum and cerium ions in total amounts of 0.025 to 0.4% by weight. The capacitor according to the present invention excels, in the temperature independency of the capacitance and tan δ, etc.

BACKGROUND OF THE INVENTION

The present invention relates to SrTiO₃ -CaTiO₃ -Bi₂ O₃.xTiO₂ typesemiconducting ceramic capacitors, particularly the capacitors which aresuitable for use in circuits demanding severe temperature properties,such as a coupling or a trap circuit. In according with this invention,the ceramic capacitors are prepared from compositions consistingpredominantly of the nonstoichiometric solid solutions which are derivedfrom a mixture composed mainly of strontium titanate (SrTiO₃), calciumtitanate (CaTiO₃), bismuth oxide (Bi₂ O₃) and titanium oxide (TiO₂) andin which minor proportions of suitable amounts of at least one memberselected from the group consisting of manganese, cobalt, nickel,chromium, vanadium, niobium, tantalum, lanthanum and cerium ions areadded thereto. This mixture is then fired in an oxidizing atmosphere toproduce a ceramic and the resulting ceramic is heated in a reducingatmosphere and some amount of oxygen eliminated therefrom to produce thenonstoichiometric solid solutions. The resulting nonstoichiometric solidsolutions are partially re-oxidized in an oxidizing atmosphere atelevated temperature to produce the semiconducting ceramic capacitors.

Although semiconducting ceramic capacitors are a relatively new field oftechnology, it is now recognized they they are superior to conventionalinsulating ceramic capacitors since they exhibit a large capacitance,are small in size and relatively compact, and exhibit other excellentcharacteristics. Semiconducting ceramic bodies for use as capacitors areclassified into two types, viz, the valence control type and thereduction-reoxidation type, according to their composition and themethod of their production. The semiconducting ceramic bodies of thevalence control type are composed predominantly of barium titanate towhich minor amounts of other elements, which have an ionic radiussimilar to those of the constituents of barium titanate but with adifferent valency, are added thereto. Since the characteristics of thesevalence control type semiconductors are strongly affected by the purityof raw materials, the maintenance of said purity during themanufacturing process, and the necessity of accurately weighing the rawmaterials in order to combine them in suitable proportions, make itdifficult, if not impossible, to produce such ceramics on an industrialscale. In fact, it is difficult to prepare such ceramics in thelaboratory, let alone on an industrial scale. In addition, the valencecontrol type semiconductors have other defects in that their specificresistivity cannot be lowered below 10 ohm-cm, and their electricalproperties are intrinsically fixed so that the temperature dependence oftheir capacitance cannot be changed arbitrarily.

On the other hand, the capacitors made from ceramics of thereduction-reoxidation type are free of the defects peculiar to thevalence control type ceramics but have other difficulties. For example,capacitors of this type, generally have such defects that the insulationresistance shows a sharp fall when the applied voltage is increased and,therefore, their working voltage in usual practical applications isgenerally 10 volts and the upper limit is fixed at about 12 volts.Another shortcoming of these capacitors is that undesirable changes ofthe electrical properties occur when lead wires are directly soldered tosilver electrodes because it is difficult to stabilize the barrierlayers. To prevent this, lead wires are usually attached to the silverelectrodes with conductive adhesives. But in practical use, when saidcapacitors are connected in a circuit, their lead wires are heated to anelevated temperature during the soldering processes, which sometimesleads to damage of the conductive adhesives. Particularly inminiaturized electric circuits, in which lead wires are short, verycareful treatment is needed.

In short, the prior semiconducting ceramic capacitors, the mainconstituent of which is barium titanate (BaTiO₃), possess no electricalproperties of barium titanate, that is, its temperature independency ofthe capacitance and tan δ, and the high dielectric constant.

Further, because of their low breakdown voltage, these capacitors havebeen limited in their utility as stated above.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide semiconductingceramic capacitors free from the aforementioned defects and whichceramic capacitors exhibit a much lower tan δ, a better temperatureindependency of tan δ and capacitance C and a higher breakdown voltagethan those of conventional semiconducting ceramic capacitors.

A further object of the invention is to provide semiconducting ceramiccapacitors excellent in the bias characteristics frequencycharacteristics and aging characteristics of capacitance and tan δ.

A still further object of the invention is to provide semiconductingceramic capacitors having stable barrier-capacitive layers formed from areduction-reoxidation process in which lead wires can be directlysoldered to silver electrodes without deterioration of any electricalproperties or damage to the electrode connection areas, that is theircharacteristics to thermal shock are improved, whereby workingefficiency may be extremely enhanced in the manufacturing process ofcapacitors or in the assembling process of electric circuit parts usingcapacitors.

In accordance with the present invention, the semiconducting ceramiccapacitors having such excellent properties may be obtained from theceramic compositions having the composition falling within thetetragonal area A-B-C-D formed by the compositional points A, B, C and Dindicated in the following Table I in which at least one member selectedfrom the group consisting of Mn, Co, Ni, Cr, V, Nb, Ta, La and Ce ionsis added thereto.

                  Table I                                                         ______________________________________                                        Point    CaTiO.sub.3 SrTiO.sub.3                                                                              Bi.sub.2 O.sub.3 . xTiO.sub.2                 ______________________________________                                        A        98.0 wt.%   0 wt.%     2.0 wt.%                                      B        0           98.0       2.0                                           C        0           50.0       50.0                                          D        50.0        0          50.0                                          ______________________________________                                    

Usually, in accordance with conventional processes for the production ofsemiconducting ceramics the use of high purity raw materials isessential which, however, increases the manufacturing cost.

In accordance with the present invention, there has been developedsemiconducting ceramic capacitors which are excellent in electricalproperties but which are produced at a considerably reducedmanufacturing cost by using ordinary industrial raw materials.

The semiconducting ceramic capacitors of this invention may be preparedby the following procedures.

SrTiO₃, CaTiO₃, Bi₂ O₃, TiO₂ and one or more members selected from thegroup consisting of Mn, Co, Ni, Cr, V, Nb, Ta, La and Ce ions areweighed in such amounts as to give a desirable composition. They areplaced in a porcelain pot on a trommel and mixed together.

The resulting raw material mixture is mixed with binders. The obtainedmixture is pressed and shaped into disks and the like. The shaped bodiesare sintered in an oxidizing atmosphere to produce ceramics, andafterwards, the ceramic bodies are fired again in a reducing atmosphere.In the latter process, the ceramic bodies lose some amount of oxygen andbecome semiconductive. Then, the semiconductive ceramic bodies thusobtained are equipped with silver electrodes and fired in an oxidizingatmosphere. This final heat treatment serves for the plating of thesilver electrode, the surface diffusion of the electrode materials onthe body and partial reoxidation of the ceramic bodies at the same time,whereby semiconducting ceramic capacitors are obtained. All of theprocesses described above are included in the manufacturing method ofthe semiconducting capacitors of this invention and are indispensable toprovide ceramic bodies with the features and properties described.

The raw materials usable for the present invention may be oxides orcompounds which give oxides by heating like carbonates, nitrates,hydroxides, hydrates and so forth.

The manganese ions may be added as manganese carbonate, sulfate,nitrate, hydroxide or manganese salts of organic acids.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further illustrated in conjunction with theannexed drawings, in which:

FIG. 1 graphically shown the variation of the capacitance and tan δ ofsemiconducting ceramic capacitors when the D.C. voltage is applied;

FIG. 2 illustrates the relationship between the variation of tan δ andthe applied frequency for semiconducting ceramic capacitors;

FIG. 3 graphically shows the aging dependence of capacitance and tan δ,respectively, of semiconducting ceramic capacitors;

FIG. 4 depicts the temperature dependence of capacitance and tan δ,respectively, of semiconducting ceramic capacitors;

FIG. 5 illustrates the relationship between the variation ofcapacitance, T.C.(%) and tan δ and the value of x in the Bi₂ O₃.xTiO₂component for semiconducting ceramic capacitors, and;

FIG. 6 is a triangular compositional diagram of the ternary systemCaTiO₃ -SrTiO₃ -Bi₂ O₃.3TiO₂ in which the contour lines of capacitance,T.C.(%) and tan δ are respectively depicted.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The raw materials are combined and mixed so as to give the composition:SrTiO₃ 66.0 wt.%, CaTiO₃ 27.0 wt.%, Bi₂ O₃.3TiO₂ 7.0 wt.% and Mn ions0.05 wt.%. The resulting materials are wet mixed in a porcelain potusing agate balls. The SrTiO₃ and CaTiO₃ are prepared in advance fromcombinations of strontium carbonate (SrCO₃) and titanium dioxide (TiO₂),and calcium carbonate (CaCO₃) and titanium dioxide (TiO₂), respectively,by calcining the equimolar mixture at a temperature of 1200° C andsubsequently roughly crushing the calcined mixture.

The Mn ions were added as manganese carbonate.

After wet mixing, binders are added to the mixture and sufficientlymixed in a ball mill. The obtained mixture is pressed and shaped intodisks of a diameter of 16.5 mm and of a thickness of 0.6 mm under apressure of 3 ton/cm².

The shaped bodies are sintered at 1350° C in air for 2 hours, andafterward fires again at 850° C in a reducing atmosphere, e.g. ahydrogen gas flow for 1 hour. In the latter treatment, the ceramicbodies loses some amount of oxygen and becomes semiconductive. Then,both surfaces of the obtained semiconductive ceramic bodies are paintedwith silver electrode paste. These ceramic bodies are heated in anoxidizing atmosphere, e.g., air, at 780° C to effect partial reoxidationof the ceramic bodies and thus the painted silver electrode material isplated on the faced of the ceramic bodies. Finally, lead wires aredirectly soldered on the surface of silver electrodes by immersing themin fused solder. The characteristic values of the semiconducting ceramiccapacitors thus obtained are as follows:

    ______________________________________                                        Capacitance per unit area                                                                            0.04 μF/cm.sup.2                                    Tan δ            0.5%                                                   Rate of change in capacitance                                                 at a temperature of -55 and +120° C                                                           -55° C; +6.2%                                   based on the capacitance at 20° C                                                             +120° C; -5.6%                                  Insulating resistance (IR)                                                    (measured at DC 100 V and 60 seconds)                                                                5 × 10.sup.4 M Ω                           Breakdown voltage      800 V.sub.DC                                           ______________________________________                                    

In the above-mentioned measurement, the values measured 24 hours afterthe reoxidation are taken as the initial values and both the capacitance(C) and the dielectric loss (tan δ) are measured at a frequency of 1KHz, a voltage of 1 v and a temperature of 25° C.

Further, the variation of the capacitance and tan of the semiconductingceramic capacitors in which the D.C. voltage was applied is shown inFIG. 1 and the frequency characteristics of the tan δ of thesemiconducting ceramic capacitors are shown in FIG. 2. Also, the agingcharacteristics of the capacitance and tan δ of the semiconductingceramic capacitors are illustrated in FIG. 3, where the horizontal axisshows the time (in hours).

From FIG. 1, it is apparent that no variation in the capacitance and tanδ occurs although the D.C. voltage is applied. As seen in FIG. 2, thesemiconducting ceramic capacitors exhibit a quantity of electricity Qnot less than 100 (the tan δ is not greater than 1%) at a frequency inthe range of from 1 KHz to 10 MHz. Also, as seen from FIG. 3, the valuesof the capacitance and tan δ are identical to their initial values evenafter the lapse of 1000 hours.

Table II shows the electric properties of the conventionalreduction-reoxidation type BaTiO₃ semiconducting ceramic capacitors andthe semiconducting ceramic capacitors according to the presentinvention.

                                      Table II                                    __________________________________________________________________________                              T.C. (%)                                                                             Bias                                                                   at a temp-                                                                           charac-                                               C                erature of                                                                           teris-                                                (μF/                                                                           tanδ                                                                        IR       -30° C and                                                                    tics                                         No.                                                                              Specimen                                                                            cm.sup.2)                                                                         (%) (MΩ)                                                                        (V.sub.DC)                                                                         +85° C                                                                        (%)                                          __________________________________________________________________________    1  BaTiO.sub.3 -                                                                       0.18                                                                              5.4 2 ×                                                                         500  -8 --  -40                                             Bi.sub.2 O.sub.3                                                                            10.sup.3 -25                                                 2  BaTiO.sub.3 -                                                                       0.17                                                                              2.0 1 ×                                                                         1200 -29 -  -40                                             La.sub.2 O.sub.3                                                                            10.sup.5 -67                                                 3  Invention                                                                           0.04                                                                              0.5 5 ×                                                                         800  +4 -   0                                                             10       -1                                                  __________________________________________________________________________

T.C. in the above table is a value calculated from the followingrelationship.

    T.C. (%) = ΔC/C.sub.20 × 100 = (C.sub.x = C.sub.100)/100

where C_(x) is the capacitance at x° C (in this case, x is -30° C and+85° C), and C₂₀ is the capacitance at 20° C.

FIG. 4 shows the temperature dependence of the capacitance (C) and tan δof the samples indicated in Table II. In FIG. 4, a solid line representsT.C. (%), while a dotted line represents tan δ and numbers 1, 2 and 3 inFIG. 4 correspond to those in the Table II. From Table II and FIG. 4, itis apparent that the semiconducting ceramic capacitors of the presentinvention are far more excellent in tan δ and T.C. (%) than theconventional semiconducting ceramic capacitors. also, Table II and FIGS.1, 2 and 3 clearly demonstrate that the bias characteristics, frequencycharacteristics and aging characteristics of the semiconducting ceramiccapacitor of the present invention are all excellent.

As can be seen from the foregoing, the present invention providessemiconducting ceramic capacitors which suffer from no disadvantagesencountered in the conventional reduction-reoxidation type BaTiO₃semiconducting ceramic capacitors and are very stable in temperature,voltage, frequency and aging characteristics and thus useful for use inelectronic circuits.

In the above Table II, the specimen No. 1 is measured at a voltage of0.5 v and a frequency of 1 KHz for the tan δ and T.C. (%) and the IR ismeasured at a voltage of 25 V. Further, the bias characteristic is arate of change in capacitance when a D.C. voltage of 25 V is applied.The specimen No. 2 is determined at a voltage of 1 V and a frequency of1 KHz for the tan δ and T.C. (%), and the IR is determined at a voltageof 50 V. Further, the bias characteristic is a rate of change incapacitance when a D.C. voltage of 50 V is applied.

Also, the specimen No. 3 according to the present invention isdetermined under the same conditions as those used in determining thespecimen No. 1.

Table III below, shows the data of electrical properties obtained byvarying the amount of Mn ion in the aforementioned composition, whichrepresents the effect of the added Mn ions.

                  Table III                                                       ______________________________________                                              Mn ion    C         tan δ                                                                         IR      V                                     No.   (wt.%)    (μF/cm.sup.2)                                                                        (%)   (MΩ)                                                                            (V.sub.DC)                            ______________________________________                                        4     0         0.040     21    1 × 10.sup.4                                                                    250                                   5     0.025     0.044     0.7   1 × 10.sup.4                                                                    600                                   6     0.05      0.040     0.5   5 × 10.sup.4                                                                    800                                   7     0.15      0.034     0.4   5 × 10.sup.4                                                                    1200                                  8     0.40      0.021     0.4   7 × 10.sup.4                                                                    1600                                  9     0.60      0.0007    0.1   ∞ 7000                                  ______________________________________                                    

As seen from Table III, the addition of Mn ions, even in a small amount,brings a rapid increase in the insulation resistance IR. This increaseis caused by the lowering of the conduction electron density in thebarrier layer owing to the valency compensation effect of Mn ions in thebarrier.

In Table III, manganese carbonate (MnCO₃) is used as a source of Mn ionsand is added in such amounts as to give a predetermined content of Mnions.

In the case where the amount of added Mn ions is less than 0.025 wt. %,the ceramic bodies produced have spots and pinholes on their surface, donot have fine surfaces and, further, exhibit a large variation incharacteristic values. On the other hand, when the amount of the addedMn ions exceeds 0.60 wt.%, the capacitance becomes quite small, althoughthe insulation resistance is high. This is because the reoxidationphenomenon during the reoxidation process becomes violent, and resultsthe deterioration of the semiconducting capacitors.

The effect of the addition of Mn ions described above in similar at anypart of the composition region A-B-C-D depicted in FIG. 6. Consequently,the amount of Mn ions to be added is limited within the range from 0.025to 0.40 wt.%

Other experiments indicate that the addition of Co, Ni, Cr, V, Nb, Ta,La and Ce ions, rather than Mn ions, each provide entirely similarresults to those obtained with Mn ions and that the combined addition oftwo or more ions selected from these ions may provide goodsemiconducting ceramic capacitor, the characteristics of which are thesame as that of the present invention, provided that the total amount ofthe added ion is within the range from 0.025 to 0.40 wt.%.

FIG. 5 graphically illustrates the relationship between the variation ofcapacitance (C), the rate of temperature dependency in capacitancemeasured at -30° C and tan δ, vs. the value of x in Bi₂ O₃.xTiO₂, whichis one of the main components of the ceramic composition according tothe present invention.

In the case where x is less than 0.5, the Bi₂ O₃ becomes excessive inamount and the resulting composition is outside the mentionedcomposition region due to the volatilization of Bi₂ O₃ during firing,which brings an undesirably violent variation in characteristics forpractical purposes. On the other hand, when x is greater than 9.0, theTiO₂ becomes excessive and the ceramic bodies adhere to each other.

In the case where x is in the range from 0.5 to 9.0, as shown in FIG. 5,the resulting semiconducting ceramic capacitors exhibit goodcapacitance, T.C. (%) and tan δ characteristics. Accordingly, in theformula Bi₂ O₃.xTiO₂, the preferred values of x are within the rangefrom 0.5 to 9.0.

Additionally, in FIG. 5, the values of x in Bi₂ O₃.xTiO₂ were variedwith the ceramic composition consisting of SrTiO₃ 66.0 wt. %, CaTiO₃27.0 wt.%, Bi₂ O₃.xTiO₂ 7.0 wt.% and Mn ions 0.05 wt.%.

FIG. 6 shows the contour lines of capacitance (C), T.C. (%) at atemperature of -25° C to 85° C and tan δ of the semiconducting ceramiccapacitors prepared from the composition as previously stated with thevariable of three elements, SrTiO₃, CaTiO₃ and Bi₂ O₃.3TiO₂.

As is also seen from FIG. 6, the semiconducting ceramic capacitors areexcellent in T.C. (%) and tan δ properties. The excellent properties canbe attained by the ceramic compositions falling within the tetragonalarea A-B-C-D in FIG. 6 in which 0.025 to 0.4 wt.% of at least one memberselected from the group consisting of manganese, cobalt, nickel,chromium, canadium, niobium, tantalum, lanthanum and cerium ions, isadded thereto. That is, the semiconducting ceramic capacitors havingexcellent properties may be obtained from the ceramic compositionshaving the composition falling within the tetragonal area A-B-C-D formedby the compositional points A, B, C and D indicated in Table IV, below,in which at least one member selected from the group consisting of Mn,Co, Ni, Cr, V, Nb, Ta, Lz and Ce ions is added thereto.

                                      Table IV                                    __________________________________________________________________________    Point  CaTiO.sub.3                                                                           SrTiO.sub.3                                                                          Bi.sub.2 O.sub.3 . xTiO.sub.2                           __________________________________________________________________________    A      98.0 wt.%                                                                             0 wt.% 2.0 wt.%                                                B      0       98.0   2.0                                                     C      0       50.0   50.0                                                    D      50.0    0      50.0                                                    __________________________________________________________________________

In FIG. 6, the value of x in the Bi₂ O₃.xTiO₂ is equal to 3, but this isonly one representative example. The semiconducting ceramic capacitorsobtained from the ceramic compositions in which the values of x in theBi₂ O₃. xTiO₂ range from 0.5 to 9.0 exhibit the same excellentproperties as those obtained by the ceramic bodies in which the value ofx in the Bi₂ O₃.xTiO₂ is 3, as is shown in FIG. 5.

In the case where the amount of the Bi₂ O₃.xTiO₂ is less than 2.0 wt.%in the ceramic bodies according to the present invention, it isdifficult to cause the oxygen contained in the ceramic bodies toliberate even if the ceramic bodies are subjected to heat treatment in areducing atmosphere, whreby they are not rendered semiconductive. Thisindicates that reduced amounts of bismuth inhibit a smooth liberation ofoxygen and reoxidation. On the other hand, when the amount of the Bi₂O₃.xTiO₂ is more than 50.0 wt.%, it is difficult to sinter the shapedbodies which is disadvantageous from the practical point of view.

Accordingly, the preferred amount of the Bi₂ O₃.xTiO₂ is in the rangefrom 2.0 to 50.0 wt.%. The amount of CaTiO₃ and SrTiO₃ corresponds tothe remainder from which the Bi₂ O₃.xTiO₂ is removed. In other words,the amount of the CaTiO₃ is in the range from 0 to 98 wt.%. In thiscase, when the CaTiO₃ is 0 wt.%, the SrTiO₃ amounts to 98 wt.%, whilewhen the CaTiO₃ is 98 wt.%, the SrTiO₃ amounts to 0 wt.%. The CaTiO₃ andSrTiO₃ are added in a weight ratio such that the desirablecharacteristics of C, tan δ and T.C. (%) may be obtained.

In the case where the total amount of the SrTiO₃ and CaTiO₃ is less than50 wt.%, sintering becomes difficult because of the increased amount ofthe Bi₂ O₃.xTiO₂ which is disadvantageous from the practical point ofview. On the other hand, when the total amount of the SrTiO₃ and CaTiO₃is more than 98 wt.%, the amount of the Bi₂ O₃.xTiO₂ is below 2 wt.%which makes it difficult to liberate the oxygen contained in the ceramicbodies by heat treatment in a reducing atmosphere, whereby the ceramicbodies are not rendered semiconductive.

As previously described in detail, the semiconducting ceramiccapacitors, which are prepared by sintering the compositions accordingto the present invention in an oxidizing atmosphere followed by heatingin a reducing atmosphere in order to liberate some amount of oxygen fromthe ceramic bodies and render then semiconductive, painting the surfacesof the obtained semiconductive ceramic bodies with silver electrodepaste and heating these ceramic bodies in an oxidizing atmosphere toeffect simultaneously the plating of the silver electrode, the surfacediffusion of the electrode materials and partial reoxidation of thesurfaces of the ceramic bodies, have a less value of tan δ, a lesstemperature dependency of capacitance and a higher insulating resistancevs. the applied voltage than the capacitors provided by the prior arts.Consequently, the semiconducting ceramic capacitors according to theinvention may be used in many fields including circuits of high workingvoltage.

Further, the semiconducting ceramic capacitors according to the presentinvention suffer from no disadvantages which accompany the conventionalreduction-reoxidation type BaTiO₃ semiconducting ceramic capacitors andhave advantages in that they possess good bias and frequencycharacteristics and their electrical properties are not influenced byaging. As a consequence, such semiconducting ceramic capacitors may alsobe suited for use in electronic circuits.

In addition, the possibility of direct bonding of lead wires to thesurface of electrodes by soldering enhances not only workability in theproduction of the semiconducting ceramic capacitors and in the assemblyof electronic circuits, but also provides good performance andsimplification of production when these semiconducting ceramiccapacitors are used in miniaturized electric circuits.

What we claim is:
 1. A reduction-reoxidation type semiconducting ceramiccapacitor comprising a substrate, a pair of silver electrodes platedthereon and a pair of lead wires electrically connected to saidelectrodes, characterized in that said substrate is an oxygen depletivesemiconducting composition consisting essentially of a solid solution ofcalcium titanate, strontium titanate and a compound represented by theformula Bi₂ O₃.xTiO₂ (0.5 ≦ x ≦ 9.0) wherein said substrate is made fromthe following raw material mixture

    ______________________________________                                                SrTiO.sub.3                                                                             66.0 wt.%                                                           CaTiO.sub.3                                                                             27.0 wt.%                                                           Bi.sub.2 O.sub.3 . 3TiO.sub.2                                                            7.0 wt.%                                                   ______________________________________                                    

and containing at least one member selected from the group consisting ofmanganese, cobalt, nickel, chromium, vanadium, niobium, tantalum,lanthanum and cerium ions in total amounts of 0.025 to 0.4% by weight,and combining oxygen on to the surface of said substrate to form anoxidation type dielectric layer thereon.
 2. A reduction-reoxidation typesemiconducting ceramic capacitor according to claim 1, wherein thesubstrate is made from the following raw material mixture:

    ______________________________________                                                SrTiO.sub.3                                                                             66.0 wt.%                                                           CaTiO.sub.3                                                                             27.0 wt.%                                                           Bi.sub.2 O.sub.3 . 3TiO.sub.2                                                            7.0 wt.%                                                           Mn        0.05 wt.%                                                   ______________________________________                                    